Flask 1.1.x reverse proxy fix path prefix

nginx.conf

	location /flask {
		proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
		proxy_set_header X-Forwarded-Proto $scheme;
		proxy_set_header X-Forwarded-Host $host;
		proxy_set_header X-Forwarded-Port $server_port;
		proxy_set_header X-Forwarded-Prefix /flask;
		proxy_pass http://localhost:4000;
	}

snippet.py

from werkzeug.middleware.proxy_fix import ProxyFix
class WsgiFlaskPrefixFix(ProxyFix):
        def __init__(self, flaskapp):
                super().__init__(flaskapp.wsgi_app, x_host=1, x_port=1, x_prefix=1)
                self.flaskapp = flaskapp

        def __call__(self, environ, start_response):
                prefix = environ.get('HTTP_X_SCRIPT_NAME') or environ.get('HTTP_X_FORWARDED_PREFIX')
                if prefix:
                        self.flaskapp.config['APPLICATION_ROOT'] = environ['SCRIPT_NAME'] = prefix
                        environ['PATH_INFO'] = environ['PATH_INFO'][len(prefix):]
                return super().__call__(environ, start_response)

application = WsgiFlaskPrefixFix(app) #declared wsgi callable